FIG. 1 is an illustrative diagram schematically showing pixels of a liquid crystal panel used in an image display apparatus such as a liquid crystal projector or the like.
As shown in FIG. 1, each pixel of the liquid crystal panel includes pixel electrode 101, common electrode 102 opposing pixel electrode 101. Further, liquid crystal 103 is held between pixel electrode 101 and common electrode 102. Opening 104 for leading light incident into liquid crystal 103 is formed in each pixel while shade 105 for shielding light is formed between pixels. Though a transistor for applying a drive voltage in accordance with an image signal is connected to each pixel electrode 101, no transistors are illustrated in FIG. 1. The drive voltage is measured by taking the potential of common electrode 102 as a reference (0V).
When a drive voltage is applied to pixel electrode 101, a potential difference arises between pixel electrode 101 and common electrode 102, producing an electric field inside liquid crystal 103 for the potential difference. As the arrangement of molecules of liquid crystal 103 changes in accordance with this electric field (which will be referred to hereinbelow as longitudinal electric field), the amount of light incident on and transmitting through liquid crystal 103 varies so as to display an image represented by the image signal.
There are cases where a potential difference arises between pixel electrodes 101 adjacent to each other, producing an electric field inside liquid crystal 103 due to the potential difference. Since the arrangement of molecules of liquid crystal 103 also changes depending on this electric field (which will be referred to hereinbelow as transversal electric field), alignment failure of the molecules of liquid crystal 103 deviating from the ideal arrangement conforming to the longitudinal electric field may occur, possibly causing light leakage, or light to leak from pixels.
Such light leakage can be prevented if the light shielding range by shade 105 is greater than a certain extent. However, in recent years, image display apparatus have been developed that feature high luminosity, high resolution and miniaturization, and the result is tendency to make opening 104 greater. As a result, the range in which light is shielded by shade 105 becomes smaller, causing difficulties in preventing light leakage.
Now, a normally white type liquid crystal panel will be described. A normally white type liquid crystal panel is a liquid crystal panel that maximizes the amount of transmittance of light incident on liquid crystal 103 when no drive voltage is applied to pixel electrode 101.
It has been known as regards normally white type liquid crystal panels that when the drive voltage that is applied to pixel electrode 101 changes from near the minimum value to near the maximum value, light leakage occurs at the pixel of the pixel electrode 101, causing display failures such as a tailing phenomenon and the like. In a word, display failure occurs at a pixel that changes from the white image to the black image.
When the drive voltage applied to pixel electrode 101 changes from near the maximum value to near the minimum value, no display failure will occur. Also, when a drive voltage near the minimum is applied to pixel electrode 101 of a pixel in which display failure has occurred, so as to produce the white image, the display failure is resolved.
Hereinbelow, the drive voltage near the minimum value is called white side voltage, whereas the drive voltage near the maximum value is called black side voltage.
FIGS. 2A and 2B are illustrative diagrams for explaining one display failure example. In FIGS. 2A and 2B, a display image at a certain point of time when a white image triangular object is moving in a black image background is shown. Here, it is assumed that the object is moving from right to left in the drawing.
In this case, the normal display image free from display failure is given as the display image shown in FIG. 2A. However, since each pixel on the trace of the object changes from the white image to the black image, light leakage takes place. Accordingly, in each pixel on the trace of the object, the background of the black image cannot be correctly displayed, causing a tailing phenomenon, as shown in FIG. 2B.
In the above way, a tailing phenomenon occurs when an object of the white image moves in the black image; there are more occasions that objects of the white image move in the black image as the area of the black image is larger, hence display failures such as a tailing phenomenon and the like become more prone to occur. Further, since the smaller the area of the white image tone, the fewer will be the pixels that are display failure is unlikely to be resolved.
In order to suppress display failures such as a tailing phenomenon of this kind and the like, there is a known method of limiting the upper limit of the signal level of the image signal.
FIG. 3A is a waveform diagram showing a drive voltage when the upper limit of the signal level of the image signal is not limited. FIG. 3B is a waveform diagram showing a drive voltage when the upper limit of the signal level of the image signal is limited. Here, the image signal uses a 1H reversing drive mechanism in which the polarity is reversed every one horizontal period (1H). Also, the image signal indicates the white image.
When the upper limit of the signal level of the image signal is not limited, pixel electrode 101 is applied with the white side voltage as the drive voltage as shown in FIG. 3A, hence there is a possibility of display failure taking place. To deal with this, the upper limit of the signal level of the image signal is limited so that the drive voltage will not fall in the white side voltage, as shown in FIG. 3B. As a result, there occurs no change from the white side voltage to the black side voltage, thus making it possible to suppress display failure.
However, in the method of limiting the upper limit of the signal level, the drive voltage does not take a value around the minimum value, so that it is impossible to maximize the amount of transmittance of the light incident on liquid crystal 103. This means that the brightness of the display image cannot be maximized, hence causing the problem of the display image darkening.
Disclosed in Patent Document 1 is a liquid crystal television apparatus that can suppress display failures and darkening of the display image.
This liquid crystal television apparatus detects the average brightness of the image signal and increases the upper limit of the signal level of the image signal when the average brightness is equal to or greater than a predetermined threshold.
With this, the upper limit of the signal level becomes lower when the black image is predominant and hence display failure is likely to occur, so that display failure can be suppressed. On the other hand, the upper limit of the signal level becomes higher when the black image is not predominant and hence display failure is unlikely to occur, so that the display image becomes bright. Accordingly, it is possible to suppress occurrence of display failure and the display image from darkening.    Patent Document 1: JP2005-6038A.